Scanning apparatuses are widely used for scanning images of documents. The scanned images can be converted into electronic files, which are then stored, processed or spread. With increasing development of scanning technologies, the scanning apparatuses have experienced great growth and are now rapidly gaining in popularity.
In the early stage, the scanning apparatus could scan one side of the document. For scanning both sides of the document, after one side of the document has been scanned, the document should be manually turned over so as to scan the other side of the document. As known, the process of manually turning over the document is troublesome. Recently, an automatic document feeder is usually integrated into the scanning apparatus. After a stack of documents to be scanned are placed on the sheet input tray of the automatic document feeder, the sheet-feeding mechanism of the automatic document feeder will successively transport the sheet to perform the scanning operation without the need of manually turning over the documents. This means of automatically feeding the sheet sheets is both time-saving and efficient. It is found that the scanning apparatuses are improved according to the generation development and the diverse demands.
Generally, a scanning apparatus has a calibration plate for performing a calibrating operation to correct the errors resulted from the optical and electronic system of the scanning apparatus before a scanning operation is performed. The optical and electronic system includes for example a light source, a lens and an optical sensing element. The calibrating operation includes for example a color calibrating operation, a brightness calibrating operation or a contrast calibrating operation.
FIG. 1 is a schematic view illustrating a conventional duplex scanning apparatus. As shown in FIG. 1, the duplex scanning apparatus 1 comprises an automatic document feeder 10 and a flatbed scanner 11. The automatic document feeder 10 comprises a first scanning module 101, a sheet input tray 102 and a sheet ejecting tray 103. The flatbed scanner 11 comprises a second scanning module 111, a glass platform 112, a first calibration plate 113, a second calibration plate 114 and a transmission mechanism 115.
Take the color calibrating operation for example. The first calibration plate 113 and the second calibration plate 114 are standard white reference plates or standard black reference plates. For example, as shown in FIG. 1, the first calibration plate 113 and the second calibration plate 114 are standard white reference plates. Before the duplex scanning operation is performed, a document (not shown) is placed on the sheet input tray 102 of the automatic document feeder 10. Then, a light beam emitted from a linear light source (not shown) of the first scanning module 101 is projected on the first calibration plate 113. As such, a standard white value related to the first calibration plate 113 is acquired by the duplex scanning apparatus 1. The standard reference value is used as the standard of white color. Similarly, the second scanning module 111 that is connected with the transmission mechanism 115 is moved by the transmission mechanism 115 to the location under the second calibration plate 114. Then, a light beam emitted from a linear light source (not shown) of the second scanning module 111 is projected on the second calibration plate 114 through the glass platform 112. As such, a standard white value related to the second calibration plate 114 is acquired by the duplex scanning apparatus 1. By using the first calibration plate 113 and the second calibration plate 114 as the standard white reference plates, the duplex scanning apparatus 1 could perform calibrating compensation for the image to be scanned.
The process of performing the calibrating compensation by the duplex scanning apparatus 1, however, still has some drawbacks. For examples, due to the limited intensity of the light beam, the effective projecting distance of the light beam is restricted. During the color calibrating operation of the duplex scanning apparatus 1 is performed, the light beam emitted from the second scanning module 111 is propagated to the second calibration plate 114 by a first distance D1, wherein the first distance D1 is within the effective distance required for obtaining a sharp image. In other words, the result of the color calibrating operation of the second scanning module 111 is satisfactory. On the other hand, light beam emitted from the first scanning module 101 is propagated to the first calibration plate 113 by a second distance D2 longer than the first distance D1. Since the second distance D2 exceeds the effective distance required for obtaining the sharp image, the scanning quality is deteriorated. Moreover, since the intensity of the light beam emitted from both ends of the linear light source is weaker, the brightness of the light beam projected on the middle part of the first calibration plate 113 is usually higher than the brightness of the light beam projected on the both ends of the first calibration plate 113. Under this circumstance, the middle part of the first calibration plate 113 is darker than each of the both ends of the first calibration plate 113. When the brightness at the both ends of the image is compensated by the first scanning module 101, the image corresponding to the both ends of the linear light source is usually over-compensated by the duplex scanning apparatus 1. Since the image corresponding to the both ends of the linear light source is too bright, the scanning quality of the duplex scanning apparatus 1 is deteriorated. Therefore, there is a need of providing a duplex scanning apparatus for correcting the calibration error.